As will be readily understood from the following description, while the present invention was developed for use in minimizing aircraft cabin noise potentially, the invention can be used in any type of vehicle to minimize any objectionable environmental parameters, including noise, in the cabin of the vehicle created by the imbalance of the engine(s) powering the vehicle.
One of the annoyances of modern air travel is the level of noise in an aircraft cabin during flight. Annoying aircraft cabin noise comes in two forms--audible form and tactile form. The audible form of noise is the sound pressure level heard by the passenger and crew of the aircraft. The tactile form of noise is the vibration felt by passengers and crew. As used in this application the word noise is intended to cover audible noise, or tactile noise, or both.
Excessive noise levels can cause aircraft passenger and crew discomfort. One source of aircraft cabin noise is engine vibration. Engine vibration is transferred through aircraft structure into the cabin of the aircraft and manifests itself as cabin noise. In addition to causing passenger and crew discomfort, engine vibration can decrease the efficiency of an engine, significantly reduce engine life, and increase engine maintenance costs.
To fully understand engine vibration, it is necessary to understand the operation of the jet engines that power modem aircraft. Most modem commercial aircraft are powered by high-bypass jet engines. High-bypass jet engines have a large number of rotating elements. The rotating elements can be grouped accordingly to the relative speed of rotation. Some of the rotating elements form a low-speed rotating system and some of the rotating elements form a high-speed rotating system. While, during in-flight operation, both the low-speed rotating system and the high-speed rotating system can be a source of unwanted engine vibration, the primary source of passenger and crew discomfort is the low-speed rotating system.
Engine vibration is caused by an imbalance in the rotating system producing the vibration. In order to reduce structurally transmitted vibration, engine manufacturers have modified the locations where engine vibration is transferred from the rotating system causing the vibration to the air frame of the aircraft. These solutions to the engine vibration problem include the use of damped bearings and vibration isolators.
Another way of reducing structurally transmitted vibration that has been implemented by aircraft operators in the past is to balance the rotating systems of aircraft engines on a regular basis. Engine balancing is well known in the aircraft art. It involves the attachment of weights at specific locations on the rotating system to be balanced. In many respects, the balancing of a high-bypass jet engine is analogous to the balancing on an automobile tire prior to mounting the tire on an automobile. Placing weights of specific mass at specific radial locations along the axis of a rotating system considerably reduces the vibration of the rotating system and, thus, the noise created by the vibration. The specification of the location and amount of weight to be applied to the rotating system in order to balance the rotating system is referred to as the balance solution for the rotating system.
In order to determine balance solutions for the rotating systems of aircraft engines, it is necessary to obtain vibration data. Vibration data is a measure of the amount of vibration that an engine is producing at various locations as the engine is operated at various speeds. Until recently, vibration data was gathered at an engine balancing facility located on the ground. More recently, engine vibration data has been gathered during flight. Regardless of how gathered, after vibration data is obtained, the vibration data is used to obtain a balance solution that attempts to minimize the vibration of the engine producing the data.
Unfortunately, all of the prior art methods used to obtain balance solutions operate under the assumption that minimizing engine vibration will also minimize cabin noise. This assumption is flawed for two reasons. First, only two locations are monitored on current engine designs. Many more than two locations would be required to cover all of the load paths an engine can use to transmit energy into the cabin. Minimization of vibration levels at only two locations does not necessarily mean that an engine is considered well balanced. Second, unbalances can lie along the interior length of the low rotor at planes that are not coincident with the fan and the last stage of the turbine. These unbalances can be due to interior blade unbalances in the engine stack up and also to rotor shaft coupling and bearing misalignments. It is not possible to completely balance an engine with access to only the exterior of the engine (i.e., the fan and last stage of the turbine). Engine balancing, therefore, is always a compromise because different balance solutions have different effects on vibration at different engine speeds, and at different locations. The criteria for success in balancing depends on how much of the dynamic picture of an engine one chooses to view. Because of the foregoing and other dynamic factors, minimizing engine vibration does not always directly correlate with minimizing aircraft cabin noise.